A method is proposed to facilitate the creation of normalized and subtracted cDNA populations. The method is based on adapters that can be protected from subsequent cleavage by the presence of modified nucleotides introduced during PCR. By protecting "tester:tester" DNA hybrids from cleavage, but not protecting "test:driver" or "drive: driver" hybrids, unwanted species can be removed from the population of amplifiable molecules. The method does not require physical separation methods, and relies only on "user friendly" laboratory procedures. The Phase I goal is to optimize several variables in the procedure so that a subtraction/normalization kit and associated cDNA libraries can be developed during Phase II.